The present invention relates to an aluminum solid electrolytic capacitor using an organic electrically conducting polymer as the solid electrolyte, and a method for producing the capacitor. More specifically, the present invention relates to an aluminum solid electrolytic capacitor, in which the pore distribution (e.g., pore volume, average pore size) of the dielectric film is adjusted by removing burrs of the chemically formed aluminum substrate cut into a predetermined shape and dissolving a part of the aluminum oxide dielectric film on the substrate surface. As a result, the organic electrically conducting polymer provided thereon is improved in adhesive property. The present invention also relates to a method for producing the capacitor and a chemically formed aluminum substrate for aluminum solid electrolytic capacitors.
A solid electrolytic capacitor using an electrically conducting polymer has a fundamental structure so that a high-density and uniform oxide dielectric film is formed on the surface of a valve-acting metal, such as aluminum, tantalum or titanium, previously subjected to an etching treatment to roughen the surface, an electrically conducting polymer as a solid electrolyte is formed on the oxide dielectric film, an anode lead wire is connected to the anode terminal (metal surface area having no solid electrolyte) of the valve-acting metal, and a cathode lead wire is connected to the electrically conducting layer containing an electrically conducting polymer, and the entire structure is molded with an insulting resin such as epoxy resin.
Among the valve-acting metals, aluminum is advantageous because the surface area can be easily expanded by an etching treatment, and the oxide film formed on the surface by an anodization treatment (chemical forming treatment) using the aluminum as the anode can be used as a dielectric film. Therefore, a capacitor having a smaller size and a larger capacitance can be produced at a lower cost compared with other capacitors. Because of these reasons, the aluminum solid electrolytic capacitor is being widely used.
The etching treatment of aluminum is generally performed by an alternating current etching in an electrolytic solution containing chloride ion or the like. By this etching treatment, a large number of pores are formed on the surface and the surface area is expanded. The radius of a pore formed varies depending on the current applied and the treatment time, but is approximately from 0.05 to 1 xcexcm.
Subsequently, the surface with pores is subjected to an anodization treatment (chemical forming treatment). By this chemical forming treatment, a high-density and uniform anodic oxide film (dielectric film) having a thickness of approximately from 0.005 to 0.1 xcexcm is formed.
The thus-obtained chemically formed aluminum substrate is cut into a predetermined size for a solid electrolytic capacitor. At this time, a protruding portion (burr) remains on the edge of the cut surface but the exposed aluminum (ground metal) portion is subjected to chemical re-forming to form an anodic oxide film (dielectric film) on the cut surface part.
An electrostatic capacitance of a capacitor device is determined by the thickness of the dielectric film, the dielectric constant of the dielectric film, and the area of the dielectric film covered by a solid electrolyte (electrically conducting substance). However, the electrostatic capacitance of conventional aluminum solid electrolytic capacitors is as small as about 80% of the theoretical value of an electrostatic capacitance (C) of a chemically formed aluminum foil (C=xcex5A/t, wherein xcex5 is a dielectric constant of the dielectric aluminum oxide, A is a surface area of the dielectric layer, and t is a thickness of the dielectric layer). Moreover, the dispersion in the electrostatic capacitance of individual products is very large. These are considered to be attributable to insufficient results in the thickness and dielectric constant of the dielectric film and in the covering area and adhesion of the solid electrolyte (electrically conducting substance) to the dielectric film when a conventional chemical forming method is used.
There is another problem in that the dielectric film in the burr portion generated during cutting or in the peak portion of the cut surface generates heat due to the current concentrated during loading of a voltage and ruptures. As a result, the solid electrolyte deteriorates in capability (e.g., voltage resistance, heat resistance).
Accordingly, an object of the present invention is to provide a solid electrolytic capacitor improved in electrostatic capacitance and reduced in dispersion of capability by treating the surface of a chemically formed aluminum film so that a dielectric film with which the electrically conducting substance provided thereon can be contacted with good adhesion over a sufficiently large contact area. Another object of the present invention includes providing a method for producing this solid electrolytic capacitor.
Another object of the present invention is to provide a solid electrolytic capacitor which is free from reduction in the properties attributable to heat generation resulting from the concentration of current by removing burrs or peak portions on the cut surface (cut end) of the aluminum substrate. Another object of the present invention includes providing a method for producing this solid electrolytic capacitor.
As a result of extensive investigations to solve the above-described problems, the present inventors discovered that when a chemically formed aluminum substrate having an aluminum oxide dielectric film is treated with an aqueous acid solution to dissolve a part of the dielectric film, surprisingly, the adhesion between the solid electrolyte (electrically conducting polymer) and the substrate is enhanced. As a result, a capacitor produced has increased electrostatic capacitance and reduced dispersion in the electrostatic capacitance of individual capacitors. Furthermore, it has been verified that the burrs on the cut surface and the peak portions of the cut end are dissolved and the deterioration of properties due to heat generation resulting from the concentration of current is mitigated when the above-described treatment with an aqueous acid solution is used. Based on this knowledge, the present invention has been accomplished.
The present invention provides an aluminum solid electrolytic capacitor, a method for producing the capacitor and a chemically formed aluminum foil for aluminum solid electrolytic capacitors, described below.
(1) A method for producing an aluminum solid electrolytic capacitor, comprising cutting a chemically formed aluminum substrate having thereon an aluminum oxide dielectric film into a predetermined shape, treating the chemically formed aluminum substrate with an aqueous acid solution to dissolve a part of the dielectric film on the substrate surface, and providing an organic electrically conducting polymer as a solid electrolyte on a chemically formed aluminum substrate.
(2) A method for producing an aluminum solid electrolytic capacitor, cutting a chemically formed aluminum substrate having thereon an aluminum oxide dielectric film into a predetermined shape, treating the chemically formed aluminum substrate with an aqueous acid solution to dissolve a part of the dielectric film on the substrate surface and burrs generated during cutting, and providing an organic electrically conducting polymer as a solid electrolyte on a chemically formed aluminum substrate.
(3) The method for producing an aluminum solid electrolytic capacitor as described in 1 or 2 above, wherein the aqueous acid solution comprises an acid selected from sulfuric acid, oxalic acid, chromic acid and phosphoric acid.
(4) The method for producing an aluminum solid electrolytic capacitor as described in 3 above, wherein the aqueous acid solution is an aqueous oxalic acid solution.
(5) The method for producing an aluminum solid electrolytic capacitor as described in 4 above, wherein a concentration of the aqueous oxalic acid solution is from about 0.1 to 15% by mass.
(6) The method for producing an aluminum solid electrolytic capacitor as described in any one of 1 to 5 above, wherein the treatment with an aqueous acid solution is a dipping treatment of the chemically formed aluminum substrate.
(7) The method for producing an aluminum solid electrolytic capacitor as described in 6 above, wherein the aqueous acid solution is an aqueous oxalic acid solution having a concentration of about 0.1 to 15% by mass, a dipping time is from 20 seconds to 10 minutes, and a treatment temperature is from about 15 to 40xc2x0 C.
(8) The method for producing an aluminum solid electrolytic capacitor as described in any one of 1 to 7 above, further comprising chemically re-forming the aluminum cut surface after the treatment with an aqueous acid solution.
(9) An aluminum solid electrolytic capacitor produced by the method described in any one of 1 to 8 above.
(10) A chemically formed aluminum substrate for an aluminum solid electrolytic capacitor, comprising a chemically formed aluminum substrate having thereon a dielectric film formed by a treatment with an aqueous acid solution, and the dielectric film having an average pore radius of 1,000 xc3x85 or more and a pore volume of 1.4 cm3/g or less.
(11) An aluminum solid electrolytic capacitor using the chemically formed aluminum substrate described in 10 above.